1. Field of the Invention
The present invention relates to an integrated circuit and fabrication method. More specifically, the present invention relates to an integrated circuit including a thin film resistor constructed in a plane elevated with respect to a device plane.
2. Description of the Related Art
Typically resistors in integrated circuits are formed through a sequence including oxidation, masking, and doping steps in which a pattern is opened in a layer of oxide on a substrate surface. Resistors are often formed in the shape of dumbbells with square ends serving as contact regions with a long, thin region within which supplies resistance. The resistance of the interior long, thin region is determined from the sheet resistance of the region and the number of squares contained in the region. The number of squares is determined by dividing the length of the region by the width.
Following doping and reoxidation, contact holes are etched in the square ends to contact the resistor into the circuit. A resistor is a two-contact, no-junction device so that current flows between contacts without crossing an N-P or P-N junction.
Resistors that are doped by ion implantation have more controlled values than resistors that are formed in diffused regions. Doped resistors can be formed during any of the doping steps in a fabrication process. For example, in MOS circuits resistors are generally formed during a source-drain doping step so that the resistor has the same doping parameters, such as sheet resistance, depth and dopant quality, as the doped device. Unfortunately, since the dopants and dosages applied to form resistors are limited by the device doping parameters, the resistances that can be achieved are similarly limited.
In addition, resistors are typically space-intensive so that the formation of resistors and the resistance values of the resistors are limited by the surface area of the substrate.
What is needed is a resistor structure and technique for manufacturing resistors that supply a wide range of resistance values and improved precision for setting resistances.